Electronic timepieces of the "analog" type, i.e., having conventional rotating hands, have been developed wherein a stepping motor is accurately driven from a time based electrical driving signal. The driving signal may come from a transistor pulse-producing circuit or from an integrated circuit having a quartz oscillator time base.
A stepping motor has a rotor with a pinion connected through a gear train to drive the hands of the timepiece. The stepping motor also has a stator assembly which is attached to the frame plate of the timepiece, a coil connected to receive current pulses from an integrated circuit, and various members of magnetizable material, including a core disposed inside the coil and pole shoes defining a circular opening within which the stepping motor rotor is disposed. The pole shoes usually define diametrically opposed notches, or are offset slightly from one another, to cause the rotor to assume a stable position between steps. The proper positioning of the stator pole shoes with respect to the rotor is extremely important for proper operation of the stepping motor. Several approaches have been used to position the stator pole shoes with respect to the rotor.
One solution is to employ a "one-piece" stator member, wherein the pole shoes are connected by diametrically opposed narrow sections, such as disclosed in U.S. Pat. No. 4,398,107-Fukushima issued Aug. 9, 1983. Another example of a one-piece stator member is shown in U.S. Pat. No. 4,270,066 issued to Terade, et al on May 26, 1981. In the latter patent, a non-magnetic reinforcing ring is employed to add additional stiffness to the one-piece stator because of the narrow sections around the stator hole. The non-magnetic reinforcing ring is disposed within the hole and has an outer diameter effective to insure a snug fit, with a circumferential side wall defining the hole through the stator. The stepping motor rotor is separately mounted to rotate within the reinforced opening, and the one-piece stator member is separately secured to the movement frame plate. A similar non-magnetic reinforcing ring pressed into the opening of a one-piece stator is shown in U.S. Pat. No. 4,342,930 issued Aug. 3, 1982 to Pellaton. In this case, the reinforcing ring is further provided with a central opening which supports one of the bearings for the stepping motor rotor.
A second approach to the general problem of locating the stator pole shoes with respect to one another and with respect to the rotor involves utilization of the frame plate and/or members secured in the frame plate to locate both the rotor and the stator pieces. One such arrangement is shown in U.S. Pat. No. 3,949,251-Takatsuki issued Apr. 6, 1976, wherein a support ring containing one of the stepping motor bearings is used as a stop for positioning tips of the stator pole shoes, which are made movable for this purpose. A related approach is seen in U.S. Pat. No. 4,725,749 issued Feb. 16, 1988 to Paul Wuthrich and assigned to applicant's assignee, wherein the two stator pole shoes are disposed in recesses formed in the frame plate and properly positioned by wedges or other means causing them to abut a wall formed around an opening housing the rotor.
Yet a third approach is shown in U.S. Pat. No. 3,978,651, issued Sept. 7, 1976 to Yoshino wherein a pair of stator yokes are permanently mounted to a reference plate, the latter being separately mountable on the watch frame plate. The two stator yokes may be formed from a single element by cutting apart by sawing or the like at diametrically opposed locations after they are mounted on the reference plate.
An improved method for making a timepiece stator is disclosed in U.S. Pat. No. 4,912,832 issued Apr. 3, 1990 to Egger, et al and assigned to applicant's assignee. A unitary stator member in this case is first secured to the timepiece frame member with two diametrically opposed narrow sections spanning two holes provided in the frame member. The narrow sections are severed with a laser to provide two separate pole shoes properly aligned by having been previously secured to the frame plate as a one-piece member. While the laser-cut stator is a significant improvement, it would be desirable to nave a separate sub-assembly which provides the same advantages, but which can be prepared before assembling to the frame.
Accordingly, one object of the present invention is to provide an improved stator assembly for a stepping motor and method of making it.
Another object of the invention is to provide that improved stator sub-assembly in which two pole shoes are properly aligned with the rotor bearings in a timepiece stepping motor and which may be separately assembled to the movement frame.
Still another object of the invention is to provide an improved process for making a laser-cut stator assembly.